Ceramic micaceous material



United States Patent Ofi ice 3,189,470 Patented June 15, 1965 3,189,476 CERAMIC MIQACEUUS MATERIAL Roger A. Long, Escondido, Califi, assignor to Whittaker Corporation, a corporation or (Ialifornia No Drawing. Filed May 29, 1961, Ser. No. 113,1)68 17 Claims. (Cl. 10639) The present invention relates to the composition and preparation of a micaceous ceramic material for high temperature applications, and more particularly to a ceramic material comprising a synthetic mica filler material, and a binder material which includes a refractory metal pyrophosphate.

Heretofore, high temperature materials characterized by electrical transparency for use in radomes and antenna win dows of high speed aircraft and missiles have lacked appreciable structural integrity. The need for stronger, more temperature resistant materials has become quite critical, and various ceramic materials have been investi gated for use in aircraft and missile radomes, antenna windows and like structures.

According to the present invention, a ceramic material is provided which is particularly suited for radomes and similar types of structures in that it has appreciable strength, withstands relatively high temperatures, and is electrically transparent for substantially distortionless transmission of high frequency microwaves, being characterized by minimum energy loss and minimum deflection or boresight error.

The present ceramic material is particularly characterized by maintenance of its strength and dielectric constant through a temperature range up to approximately 1000" F. Its moisture absorption is low, and it is relatively easy to prepare and fabricate into laminated and molded structures, utilizing either cold press and sintering, hot pressing, or melt and cast techniques.

The ceramic material takes three main forms: (1) a metal pyrophosphate binder in combination with a synthetic mica flake filler material; (2) a eutectic binder made up of a metal pyrophosphate and a synthetic mica flake combined with a symthetic mica flake filler material; and (3) a eutectic binder made up of a metal pyrophosphate and a refractory oxide or oxides combined with a synthetic mica flake filler material. It is noted that in each case the filler material is constituted by synthetic mica flake or micaceous crystals, and the metal pyrophosphate is a constituent of the binder. Neither of these materials is classed as a refractory material, manganese pyrophosphate and synthetic mica having melting and decomposition points, respectively, of approximately 21:82" P. and 2350 F., it being noted that the filler always has a melting or decomposition point above that of the binder.

Metal pyrophosphates have an atomic bonding structure which imparts great strength to the ceramic material, and are desirably characterized by a predetermined melting point, and an ability to maintain hardness at elevated temperatures. Further, they have a desirably low shrinkage rate upon sintering subsequent to pressing.

Synthetic mica is used in preference to natural mica because the natural mica is not as stable at elevated temperatures, containing hydroxyl groups which form water at relatively low temperatures. The synthetic mica, on the other hand, does not include hydroxyl groups, these having been chemically replaced by fluorine which are more strongly bound compared to the hydroxyl groups of the natural mica. Consequently, the synthetic mica is much more thermally stable, not evolving Water or gases at the elevated temperatures contemplated by the present invention.

In that form of the present material which employs a eutectic binder composition of a metal pyrophosphate and a synthetic mica flake material, the initial provision of the eutectic prevents subsequent dissolving or degradation of the added mica flake filler at or near the eutectic temperature. That is, since the pyrophosphate is in eutectic form, it has already combined with all of the synthetic mica with which it is capable of combining at that specific temperature. The resulting absence of combination with or attack upon the subsequently added mica filler preserves the flake integrity of the mica filler, and, consequently, the finished ceramic material is characterized by relatively high structural strengths However, as will be seen, the flake filler is sufficiently wetted by the pyrophosphate-mica binder for good bonding, without destroying the structural strength expected of a flake type of filler.

Other objects and features of the present invention will become apparent from the description hereinafter made.

MANGANESE PYROPHOSPHATE The pyrophosph-ate employed preferably comprises manganese pyrophosphate (Mn- 11,0 but it may comprise certain other pyrophosphates which either have a melting point less than the 2350 F. decomposition point of synthetic mica, or a melting point of less than 2350 F. in eutectic form with synthetic mica or certain refractory oxides to be described in greater detail hereinafter. Such additional pyrophosphates include: titanium pyrophosphate (13 F 0 iron pyrophosphate (Fe P O zirconium pyrophosphate (Zr P O nickel pyroph ph e (Nlgpzoq); and the like.

Manganese pyrophosphate has a melting point of 2182 F is insoluble in water, and has a density of 3.707 g./ cc. It is a brownish-pink material which may be obtained by a variety of chemical processes, one sim le process eing the dehydration of hydrated manganese pyrophosphate (Mn P O -3H O) by heating to a sufficiently high temperature to drive off the water of crystallization. Another process is the calcining of ammonium manganese phosphate (NH Mn-PO to drive oif the combined water and ammonia and force the chemical rearrangement to the pyrophosphate by the following reaction:

where time and temperature are the considered variables. The process used is based on wet chemistry and the following procedure and reaction took place:

NH4OH MnCIMH O NH4H2PO4 NHm/IDPOyHgO ZHCI 31120 A ZNH4BTI1PO4-Hg0 MHQP1O7 ZEN; -I- 31120 Initially, the above two salts are dissolved independently, the two solutions then mixed and thereafter precipitated with concentrated NH OH solution. Satisfactory purity was obtained by several distilled water washings, checking the filtrate with silver nitrate solution for chlorine ion presence. Next, the washed material was calcined at 1400 for approximately one-half hour.

Calcination was carried out in a fireclay crucible in air using a furnace. The calcined material was then broken into pieces approximately /2 to A; inch, and ball milled until it would pass a 200 mesh screen.

Chemical porcelain and zircon crucibles were both satisfactory for melting the manganese pyrophosphate, it being noted that fireclay, alumina, and beryllia crucibles undesirably reacted with the manganese pyrophosphate and were, therefore, unsatisfactory.

ESTABLISHMENT OF EUTECTIC OF MANGANESE PYROPHOSPHATE AND REFRACTORY OXIDES The above-prepared pyrophosphate is used in all of the Thus, the binder is a liquid phase binder as -200 mesh GC Grade, titania (TiO chromia I (Cr O obtained as approximately -200 mesh, thoria (T110 obtained as 200 mesh, hafnia .(HfO obtainedas -200 mesh, and magnesia (MgO). The eutectic mixtures for the pyrophosphate andthe refractory oxidemay be established in a number of ways,

one satisfactory way comprising: pressing the blended pyrophosphate and oxide powders at 4000 p.s.i.into

phosphate for the several specimens; placing these specimens side by side in a zircon boat; an d heating them in a furnace at a preselected temperaturefor minutes. The specimens are then examined to determine which one of the specimens had just begun to flow or melt at the pre-selected temperature, as indicated by a slight rounding of the corners of the specimen, and that particular one of the melted specimens thereby indicates the approximate composition'of the eutectic. Next, sev-' pending on the results of the previous tests, until the lowest temperature of melting or rounding of the corners is determined. This procedure gives an approximation of the eutectic composition, and was found to be quite reliable and accurate.

It was determined that for manganese pyrophosphate I (-200 mesh), eutectic mixtures were formed at the following approximate temperatures for the approximate percentages, by weight, of the manganese pyrophosphate and the oxide:

Table l Eutectic Oxide melts Eutectic composition temperature, ing point,

7.5% A1203, 92.5% 111121 207-. 1, 987 3, 720 11.0% ZIOz, 89.0% MnzPzoL. 2, 015 4, 890

. 5.5% BeO, 94.5% MJIQPZOL 1, 858 4, 586 12.5% TiOg, 87.5% NIHZPQOT 1, 910 2, 984

p 4.75% MgO, 95.25% MmPgOv 2, 005 5, 072 7.5% C1203, 92.5% Mn1P O 1, 976 3, 614 25.0% T110 75.0% Mn2P207 1, 906 5, 522 16.5% H102, 85.5% MmPiO1 1, 980 5, 090

Of course, the percentage of the oxide in solution will increase with higher temperatures. It is noted that theeutectics set out are characterized by a melting point low enough for relatively easy preparation. and part inch diameter by /2 inch cylindrical specimens, using 'various percentagesof each of the oxides and the pyroeral' more specimens are prepared with the percentages Y of oxide of each varying by, for example, 1%, and the .:test re-run to determine which of these just melts at V {the preselected temperature. Next, further specimens "are: prepared with slight variations between them, as respects the oxide composition, and the pro-selected tem- :perature is 'then varied-upwardly or downwardly, de-

fabrication, and are yet high enough to be able to with- 1. stand temperatures approximately 2000f F. without deterioration. The above percentages and temperatures. were derived by experimental techniques and are, therefore, only close approximations of the eutectic composi- 5 tions and temperatures. 1 It was found that small percentages of certain mate- 1 rials in the pyrophosphate have a pronounced elfect on the eutectic compositions at the various temperatures, and these materials should, therefore, be kept atia minimum for uniformity of results. However, a. predetermined adjustment of such materials is another means for easily varying the eutectic melting point to tailorthe melting point to the particular application at hand.

The most active of such materials or compounds in the manganese pyrophosphate appears to be one of the sodium phosphates, which constituted less than 10% by weight of the manganese pyrophosphate used in establishing the above experimental results. The elfect of such sodium phosphates in lowering the eutectic melting point was confirmed by making additions to the pyrophosphate of between 10% and 20% by weight of Na(PO H O (sodium tri-metaphosphate monohydrate). The additions werefound to efiectively lower the eutectic temperature as expected.

A table is, set forth below to show the variations in two different batches of commercially available Mn P O due to the above-described impurities, percentages being by weight.

1 All melted at l,667 F.

PREPARATION OF 'EUTECTIC OF MANGANESE PYROPHOSPHATE AND REFRACTORY OXIDES After determining the composition of the eutectic of manganese pyrophosphatc and the refractory oxide, the preparation and powdering of the eutectic for later combination with the synthetic mica filler m'aterial was as follows. The manganese pyrophosphate (-200 mesh) was blended with the'oxide (200 mesh) from four to twelve hours in a rotating glass jar containing rubber balls. 7

The blended mixture was pressed into bricquets by a pellet press, which were thereafter melted in an eleetric furnace in air. The bricquets may also be melted in a vacuum induction furnace into which an inert gas is introduced to create a partial pressure. The induction heating of an electric furnace under vacuum eliminates the necessity of stirring because of the characteristic stirring of induction heating Zircon and graphite crucibles were 'used to contain'the material, and melting was effected by bringing the eutectic mixture to a temperature approximately 50 to 200 F. above the predetermined melting point of the, eutectic being prepared. 7

The melted mass was immediately poured into 'cold water and the resulting frit dried-and then ball milled in a rubber lined porcelain ball mill for particle reduction to at least l00 mesh or finer. Other'non-contaminating ball mills could be, used.

The combination of the above eutectic of manganese pyrophosphate and a refractory oxide with the synthetic mica filler will be described in detail hereinafter.

ESTABLISHMENT OF EUTECTIC OF MANGANESE PYROPHOSPHATE AND MlCA 'cal stability in air to about 2250 F. and is decomposed at about 23 50" F. The high temperature range of the material is apparently attributable to the fluoride replacement of hydroxyl groups which occur in the natural mica and which undesirably form gas or water at elevated temperatures. A suitable synthetic mica is sold as Synthamica 202. It is in flake or crystalline form and has a melting point of 2492 F., an apparent density of 1.7

and then placed in a kiln and subjected to various sintering temperatures from 1800 to 2400 F. for periods up to one hour. After sintering, the disks were withdrawn from the kiln and allowed to cool in air, and again weighed and measured. Slower heating and cooling 2 gmsjccmq a fecgmnlended temperature li it of 5 methods may be used if it develops that the particular 1832 F., and a tensile strength of 5000 to 10,000 psi. disk formulation is subject to failure from thermal shock. It has a dielectric constant at one megacycle of 6.3, a dis The disks which :were pressed at 8000 psi. and sintered sipation factor at one megacycle of 0.0005 to 0.0020, and at h venous temperatures were next examined r a volume i i it h f 5 10 greatest density and minimum shrinkage, these criteria 10 being used on the assumption that maximum strength is a PREPARATION OF EUTECTIC OF MANGANESE function of maximum density, and that the lower the PYROPHOSPHATE AND MICA shrinkage the better the formulation will be to facilitate part fabrication. As the following table indicates, the In preparing the eutectic synthetic mi a and ma ganes optimum binder-filler formulation for synthetic mica PY P P S for subsequent use in combination with 1 filler and the non-eutectic form of the manganese pyrothe synthetic mica filler, the composition of 5 wt. percent phosphate binder was approximately 80% filler and mica and 95 wt percent MH2P207 was prepared by blendbinder (all percentages by Weight):

Table III Composition Sintering Mod. oirup., p.s.i.

tenirifi Ngc. Youlmg Percentlin. MnzPz01 Mica Sp Ave. Max m0 Shrmk 20 so 2,000 4 3,360 3,740 0.5 20 80 2,100 4 3,190 3,940 0.25 20 80 2,190 4 1,260 1, 470 -0. 60 40 1,795 4 2, 080 2,200 0. 25 60 40 1,860 3 1,920 2, 230 2.25 60 40 1,960 4 1,940 2,550 3. 25 so 20 1,630 4 1,510 1,660 0.00 so 20 1,750 4 3,140 3,050 1.0 80 20 1,870 4 940 1,100

ing the two materials for 16 hours. The particle size of The strength data was obtained by pressing and sinterboth components was 200 mesh. ing the various formulations into bar specimens measur- Induction heating and an AGX graphite crucible were ing 3 /2 x 1% x 75 used to melt this material mer it had been pressed into It was noted during these tests that when 10-15% by bars weighing about 200 gms. each. No special atmos- 40 weight of the binder was used, the specimens sintered phere was used during the melting phase, although the poorly, had low density and low strength. When the eutectic mixture was heated as rapidly as possible to 100 binder was increased to 30% by weight, the specimens F. above its melting point. The temperature was held showed a steady drop-01f in strength with increasing sinafter melting for a period of five minutes to insure comtering temperature, and there was a consistent and undeplete solution and mixing, an optical pyrometer being r sirable tendency of the higher percentage specimens to used for the temperature determination. The melt then Warp during sintering. Thus, as previously stated, the was immediately poured into a stirred Water quench bath optimum percentage by weight for the binder falls in f f itting the approximate range of 15% to 25% and is preferably The frit was removed from the quench and washed in 20%. water. After drying, the frit was ball milled with por- The low shrinkage rate of the above 80%20% formucelain grinding media in porcelain mill jars until is passed lation is very good, and is theorized to be due to good a -200 mesh screen. The material collected was stored wetting or liquid phase sintering between the manin glass jars until needed for use. ganese pyrophosphate and the mica flakes. ESTABLISHMENT OF OPTIMUM COMBINATIONS The optimum sintering temperature for the straight p MICA FILLER WITH EUTECTIC AND manganese pyrophosphate binder, from the above Table EUTECTIC FORMS OF MANGANESE PYRO III, 1s m the approximate range of 2175 to 2200 F., in- PHOSPHATE dicating that sintering should preferably be carried out at 1 1 temperatures slightly in excess of the melting point of After preparing the two forms of eutectic, as above described,'that is, manganese pyrophosphate and an oxthe bmder' to eflect Yvettmg and hmlted Solution of ide, and manganese pyrophosphate and synthetic mica, i' finer Wimre the bmder t i t PYIOPhPSPhate each of the forms was next blended with various amounts noon'eutechc form the menu-3g P 15 appl:oxlmately of synthetic micaflake filler to determine optimum formu- 2182 However when bmder 1S a eutectlc of lations. This was done in a porcelain ball mill using ganee pyrophosphate and mlca, i meljing Point is P- spherical alumina balls, for a Period of approximately proxlrnately 1885 F. The melting points of eutectics one hour, and screened to Obtain a 100 mesh 'mix, of manganese pyrophosphate and the various refractory Approximately 1% by weight of /z 4000 cps. Methocel Oxides is Set out in Table solution was then blended into the mix to achieve granu- If sintering is carried out at temperatures appreciably lation of the ingredients and green strength. above the melting point of the binder, such as over ap- The mix was then pressed into 1" diameter by /2" proximately 200 F. higher, the mica flake fillers structhick disk cylinders at pressures of 4000, 8000 and tural integrity is destroyed by erosion, that is, by going 10,000 p.s.i., and from subsequent strength tests, it was into solution with the binder. This is to be avoided in determined that 8000 psi. gave consist ntly h gher order to preserve the function of the mica as a filler. strengths than either the 4000 or 10,000 p.s.i. compacting Limited erosion of the filler is theorized as being desirable pressures, to provide a good bond between the binder and the filler. The pressed disks were dried, weighed and measured, The dielectric data on the optimum binder and 20% mica filler of'Table III above was determined at various temperatures as follows:

A significant improvement in strength and density was noted for the optimum 80% binder-20% filler (by weight) formulation when the mica filler was employed in two particle sizes. Various percentages of relatively coarse and flne mica were used, and it was found that *the finer micron particle size provided better compressive strength, the coarser-200 mesh provided better density, and that a blend of the two was superior in both strength and density properties. 7 mica flakes fill voids existing betweenthe larger mica flakes and provide better structural continuity. The optimum mica particle formulation for the filler which constituted 80% by weight of the ceramic material was found to be approximately 35% of 200 mesh mica plusapproximately 45% of the 5 micron mica, to yieldthe 80% total. Thus, thecoarse mica constituted approximately 44% by weight, of the mica total, and the fine, mica constituted approximately 56% by weight, of

the mica total; Examples I and II below illustrate the strengths'obtained with the two particle sizes of mica filler, with the straight Mn P O 'binder, and with the eutectic of the Mn P O and mica, respectively (percentages by weight):

EXAMPLE I V r [80% syntheltiic mica (35%, -200 mesh; 45%, 5 microns), Mn P O 20O mes R.T. 600 F; 800 F. 1000 F;

Average density, gm./Cm. 2. 01 2. 02 2. 01 2. 09 Average flex strength, p.s.i V 3940 4, 020 3, 621 4, 000 Average modulus of elasticity, psL 3. 77x10 4. 38 3. 98 3. 81

7 EXAMPLE II {80% synthetic mica 200 mesh; 45% microns), 20% micaeutectic, 200 meshl V R.T. 600 F. 800 F. V 1000 F. i

' Average density, gm./cm. 2.07 2.13 2. 15 2. 09 Average flex strength, psi--- 4, 440 4, 420 4, 490 4, 090 Average modulus of elasi ticity, p.s.i 4.4000 4. 29 4; 08 r 4. 17

A typical formulation of mica-filler with a eutectic l of manganese pyrophosphate and a refractory oxide 1 (A1 0 is set forth in the following Example III, Ex-

. ample IV being MHZPZOI] without any filler for purposes of comparison (all percentages being approximate, and

j by weight):

7 EXAMPLE III Eutectic of A1 0 and MIlzPzOq percent 20 Synthetic mica do 80 I Sintering. temperature F 2100 1 Modulus of rupture p.s.i. 4500 Apparently, the finer 0 EXAMPLE i'v [100% lVImPzOfl Sintering Rs 1 temperature,

(1) Modulus of Rupture. 2, 540 1, 900 (2) Modulus of Rupture 1, 510 2, 000

From the above, it-will be seen that a ceramic material has been provided'which is characterized by dielectric properties uniquely adapting the material for employment in radomes and similar types of structures. It has low density, good thermal shock resistance, excellent strength, electrical stability over wide ranges of tempera- 'tures and frequencies, good erosionresistance, and extremely low moisture absorption. Further, the ceramic materials according to the invention retain their strength a thetic mica is defined to be mica in which the hydroxyl groups of the natural mica have been replaced by fluorides or the like to provide elevated temperature stability.

I claim:

1. A ceramic material consisting essentially of: synthetic mica flakes intimately mixed with and bonded by ,a binder consisting essentially ofa eutectic of manganese pyrophosphate and synthetic mica, said binder being. pres- 1 ent'in an amount suflicient to effectively bond said mica flakes.-

2.'A ceramic material consisting'essentially of: syn

' thetic mica flakes intimately mixed with and bonded by of a refractory oxide, said binder being present in an amount suflicient to effectively bond said mica flakes.

' mately 80% by weight of synthetic mica flakes intimately.

3. Ina ceramic material, the combination of: synthetic mica flakes intimately mixed with andbonded by a binder consisting essentially ofa eutectic of manganese. pyrophosphate and approximately 7.5% by weightof alumina,

said binder being present in an amount suflicientfto T effectively bond said mica flakes.

4. A ceramic material consisting essentially of: a synthetic mica flake filler intimately mixed with and bonded V bya binder of approximately 95% by weight of manganese pyrophosphate and approximately 5% by Weight of synthetic mica flakes, said binder being presentin an amount sufficient to effectively bond said mica flakes.

5. In a ceramic material, the combination of: approximixed with and bonded by approximately 20% by weight of a binderconsisting essentially of a eutectic approxi- .6. The method of manufacturing a ceramic material comprising: blending approximately 5% by weight of synthetic micaceous flakes with approximately 95% by.

weight of finely divided manganese pyrophosphate; press ing saidmanganese pyrophosphate and said micaceous flakes, melting said manganese pyrophosphate and said micaceous flakes; reducing the melted material to a finely divided condition; blending the material so treated with micaceous flakes constituting approximately by weight of the total blend, the melted and reduced material 7 constituting approximately 20% by weight of the total blend; pressing the material so blended at a pressure of at least 4,000 pounds per square inch; and sintering the pressed material at a temperature of approximately 1800 to 2350" F.

7. The method of manufacturing a ceramic material comprising: blending, in eutectic proportions, a pyrophosphate in a finely divided condition, selected from the group consisting of manganese, titanium, iron, zirconium, and nickel, with a material in a finely divided condition, selected from the group consisting of micaceous crystals and refractory oxides; melting the pyrophosphate and selected material; reducing the blended material to a finely divided condition; blending the material so treated with micaceous flakes in an amount suflicient to effectively bond said micaceous flakes; and sintering the resultant material above the eutectic melting temperature.

8. A ceramic material of the crystalline type adapted for use in radomes, antenna windows and the like, conessentially of an intimate substantially homogeneous mixture of micaceous crystals and a binder, said binder being present in an amount sufficient to efl ectively bond said micaceous crystals, said binder being a eutectic of a metal pyrophosphate and a member selected from the group consisting of micaceous crystals and refractory oxides, said binder having a melting point less than the decomposition point of said micaceous crystals.

9. A ceramic material of the crystalline type for use in radomes, antenna windows and the like, consisting essentially of synthetic mica crystals intimately mixed with and bonded by a binder, said binder being present in an amount sufficient to efliectively bond said micaceous crystals, said binder being a eutectic having a melting point less than the decomposition point of said crystals and consisting essentially of a metallic pyrophosphate and a member selected from the group consisting of micaceous crystals and refractory oxides.

10. The material of claim 2 wherein said refi'actory oxide is alumina.

11. The material of claim 2 wherein said refractory oxide is zirconia.

12. The material oxide is beryllia.

13. The material oxide is titania.

14. The material oxide is magnesia.

15. The material oxide is chromia.

16. The material oxide is thoria.

17. The material oxide is hafnia.

of claim 2 wherein said refractory of claim 2 wherein said refractory of claim 2 wherein said refractory of claim 2 wherein said refractory of claim 2 wherein said refractory of claim 2 wherein said refractory References Cited by the Examiner UNITED STATES PATENTS 3/55 Comeforo 106-39 8/59 Bray et al. 106-65 TOBIAS E. LEVOW, Primary Examiner.

JOSEPH REBOLD, Examiner. 

8. A CERAMIC MATERIAL OF THE CRYSTALLINE TYPE ADAPTED FOR USE IN RADOMES, ANTENNA WINDOWS AND THE LIKE, CONSISTING ESSENTIALLY OF AN INTIMATE SUBSTANTIALLY HOMOGENEOUS MIXTURE OF MICACEOUS CRYSTALS AND A BINDER, SAID BINDER BEING PRESENT IN AN AMOUNT SUFFICIENT TO EFFECTIVELY BOND SAID MICACEOUS CRYSTALS, SAID BINDER BEING A EUTECTIC OF A METAL PYROPHOSPHATE AND A MEMBER SELECTED FROM THE GROUP CONSISTING OF MICACEOUS CRYSTALS AND REFRACTORY OXIDES, SAID BINDER HAVING A MELTING POINT LESS THAN THE DECOMPOSITION POINT OF SAID MICACEOUS CRYSTALS. 